There are instances where the conversion of metal oxides to metal fluorides may be desirable. For example, it may be desirable to prepare water-soluble fluoride compounds to be employed in the treatment of water supplies and the like. Also, metal fluoride fluxes sometime become contaminated with various metal oxide materials. It may be desirable to reduce the amount of metal oxide in the flux or convert the metal oxide to a metal compound which is more compatible with the flux, for example, another metal fluoride compound.
Several methods have been employed to prepare metal fluorides. The most common method employs hydrogen fluoride as a raw material. This method requires expensive specialized equipment since hydrogen fluoride must be prepared prior to the preparation of the metal fluoride.
The aqueous HF treatment of oxides is not an effective process for treating all oxides, since with many oxides, hydrates of the fluorinated products are formed, e.g., AlF.sub.3.3H.sub.2 O, BeF.sub.2.XH.sub.2 O, ThF.sub.4.4H.sub.2 O and the like. This water cannot normally be removed without reformation of substantial quantities of the original oxides. Likewise, in the aqueous HF process several steps are required, i.e. cooling, grinding, reacting, drying and remelting to prepare purified fluorinated products.
On the other hand, the direct treatment of fused salts with gaseous HF is difficult to conduct because (1) the corrosiveness of HF.sub.(g) requires specialized equipment; (2) the reaction rate and efficiency of HF is low requiring excessive reaction times of large HF.sub.(g) losses; and (3) the fluorination reaction is usually incomplete leaving substantial quantities of unreacted metal oxides.
Several other processes have also been developed for producing metal fluoride products. The teachings of U.S. Pat. Nos. 2,690,430; 2,877,095 and 2,958,576 are exemplary of other known processes. All of these processes, however, have certain inherent disadvantages associated with them which are objects to be overcome in the instant invention. For example, CF.sub.4 is an expensive raw material. The process taught in U.S. Pat. No. 2,690,430 can be employed only to produce a mixture of fluoride and hydroxide or oxide. In the process taught in U.S. Pat. No. 2,877,095 anions are present which can contaminate the fluoride product. Moreover, aqueous conversion systems usually require large volumes of materials. High wastes are involved and the required dehydration step is often expensive. In another process, it has been taught to react lead fluoride with magnesium oxide to form lead oxide and magnesium fluoride.
The present invention offers the following advantages over those known in the art. First, the process is a solution reaction. This offers a more efficient process which goes substantially to completion. Secondly, metals which are immiscible with the metal fluoride product are produced in the reaction. Further, the use of carbon plus a fluorinating agent allows for the release of oxygen from the oxides at a lower temperature than when the oxide is reduced with carbon or the fluorinating agent is reacted with the oxide. In addition, in the present process, oxygen is removed as a gaseous oxide of carbon which is easier to handle than are oxides of metal. The present process can be employed to fluorinate refractory oxides. Further, the fluorinated product prepared by the present process is usually substantially free of metal oxide impurities.